The invention concerns a circuit layout for the elimination of alternating voltage interference signals superposed on direct voltage signals which are to be amplified. The direct voltage signals are to be amplified using a signal amplifier with low-pass characteristics to which both the alternating voltage interference signals and the direct voltage signals are passed.
The amplification of small direct voltage signals is a problem frequently arising in electrical or electronic measuring technology. In particular in remote measuring applications, because of the larger distance of the transducers to their evaluating locations and the need to pass such measuring signals over long lines to an amplifier, there is a high risk that interference signals will be superposed by induction on the measuring signal itself. The interference signals may be caused by the electromagnetic fields of the supply installation. They may also, however, be the result of inadequately filtered power supply and network devices, the interference signals appearing as so-called ripple voltages in the frequency range of 50 to 120 Hz, depending on whether the direct supply voltages are generated by half-wave or full-wave rectification in 50 or 60 Hz networks.
Since such interference signals no longer permit accurate measurements when the amplitudes of the interference signals attain or exceed those of the useful signals, means must be found to significantly weaken the interference signals with their harmful effects. It is known to use low-pass filters to suppress interference voltages in direct current lines of power units; such filters may be in the form of active or passive filters. An example of an electronic filter choke, the dc resistance of which is significantly lower than its dynamic resistance (ac resistance), is described in an article "Transistors and their applications in industry", Elektronik 1962, No. 5, page 132. Whereas passive low-pass filters require relatively large capacitors occupying much space, active low-pass filters consist of a larger number of electronic structural elements.
Like active low-pass filters, frequency dependent amplifiers are easier to provide. But they must also process interference signals, which may become a problem if the amplitude of the interference signals is several times that of the useful signals.
The harmful effects of interference signals may further be reduced by their compensation, as described for example in U.S. Pat. No. 4,129,853. Interference signals are generated by light sensitive transducers in the course of optical scanning performed on the basis of brightness fluctuations of the background and its illumination. Compensation is achieved by determining said interference signals, storing them, and subtracting them, during evaluation of the signal, from the overall signal.
Since the cost and complexity of such an approach is very high, in cases were a moderate technical investment and a high degree of reliability in the suppression of interference signals are required, such an approach is disadvantageous.